Improve comments in .env.base file

Created by: player1537

Description

The base environment file should be made more descriptive, so that it includes better descriptions of the purpose of each variable as well as a default or typical value.

Additionally, it needs to include the TEST_INTERPRETER_ARGUMENTS variable and a description of how to use the comma separated parts of the list.

Lastly, it probably shouldn't use *_INTERPRETER_* and instead *_PYTHON_* since it's only expecting a Python interpreter, which also means the name isn't needed.